Relative pressure sensor

ABSTRACT

A pressure sensor for measuring a pressure difference between a pressure being measured and the ambient atmospheric pressure surrounding the pressure sensor, including a platform and a membrane loadable with a pressure being measured. The membrane is secured at its edge to the platform, with a pressure chamber thus being formed between the platform and the measuring membrane. The pressure chamber communicates over a reference air path with the atmosphere, with the reference air path being a winding path.

FIELD OF THE INVENTION

The present invention relates to a relative, or gage, pressure sensor.Relative pressure sensors usually measure the difference between thepressure in a medium being measured and the current ambient atmosphericpressure. A relative pressure sensor includes, in general, a platform onwhich a measuring membrane, or diaphragm, is attached pressure-tightlyat its edge, with a pressure chamber being formed between the measuringmembrane and the platform. For relative pressure measurement, referenceair is introduced into the pressure chamber through an opening in theplatform, and the measuring membrane surface facing away from themeasuring chamber is exposed to the pressure being measured. Theresulting deformation of the measuring membrane is a measure of therelative pressure, and this is transduced in suitable manner into ameasurement signal.

BACKGROUND OF THE INVENTION

The mentioned introduction of the reference air can bring moisture intothe pressure chamber, which then condenses in the interior of the sensorwhen the temperature falls below the dew point. This can degrade thefunctioning of the sensor. Such is especially a problem, when the airsurrounding the sensor has a higher temperature than the medium whosepressure is being measured.

Hegner et al. disclose in European Patent Application EP 0 974 825 A3 arelative pressure sensor, which has a reference air path with a moisturefilter. The moisture filter is arranged in the front area of therelative pressure sensor, near the measuring membrane, thus near themedium, so that the temperature of the moisture filter is similar to thetemperature of the media. This arrangement assures that moisture in thereference air can condense out before it even gets into the referenceair path, so that there is hardly any possibility of a falling below thedew point in the pressure chamber. The described arrangement is,however, comparatively costly.

Other relative pressure sensors have a capillary tube communicating withthe platform-side opening of the pressure chamber. The capillary tubeserves as a reference air path, while opposing encroaching moisture witha certain diffusion resistance. The capillary tube is frequently a smallmetal tube, which, for example, is secured to the platform by glazing.This manner of assembly is accompanied, likewise, by an increasedmanufacturing cost. Additionally, the entrance opening of the capillarytube is largely thermally de-coupled from the pressure measuring cell,so that, in the case of higher temperatures at the entrance opening, airwith a high water content can get into the capillary tube, which leadsto falling below the dew point in the colder pressure chamber. Thecondensation in the measurement cell is significantly delayed and evendecreased by the described arrangement, but it cannot, however, beprevented by this technique.

SUMMARY OF THE INVENTION

It is, consequently, an object of the present invention to provide arelative pressure sensor having a simple and compact construction, whichdelays, respectively decreases or prevents, the incursion of moisturemore effectively. The object is achieved by a pressure sensor includinga platform and a measuring membrane loadable with a pressure beingmeasured, with the measuring membrane secured at its edge to theplatform, with a pressure chamber being formed between the platform andthe measuring membrane, with the pressure chamber communicating over areference air path with the atmosphere, the reference air path includinga winding path.

The pressure sensor of the invention for measuring a pressure differencebetween a pressure being measured and the ambient atmospheric pressureincludes a platform and a membrane loadable with the pressure beingmeasured. The membrane is secured at its edge to the platform, with apressure chamber in this way being formed between the platform and themeasuring membrane. The pressure chamber communicates over a referenceair path with the atmosphere, with the reference air path being awinding path.

The winding reference air path serves as a diffusion barrier and delays,in this way, the incursion of moisture into the sensor interior,especially into the pressure chamber. Since the path is winding, it ispossible to accommodate within the compact dimensions of the pressuresensor a path which is sufficiently long to attain satisfactory results.The length of the winding path amounts preferably to at least 75%, morepreferably to at least 100% and especially preferably to 150%, of thelength of the perimeter of the separating membrane. With reference tothe axial dimension of an essentially cylindrical relative pressuresensor, the winding path is preferably at least twice as long as theseparation of the atmosphere-side opening of the winding path from theplane of the measuring membrane.

In addition, the compact arrangement of the path resulting from thewinding shape offers the possibility of assuring a good thermal contactbetween the atmosphere-side opening of the reference air path and thepressure chamber.

This is advantageous insofar as it keeps the temperature of theatmosphere-side opening of the reference air path fairly close to thetemperature of the pressure chamber. Advantageously, a filter isprovided at the atmosphere-side opening of the reference air path, forassuring that no condensate can penetrate into the cell. In this way, itis practically impossible that it can fall below the dew point in thepressure chamber under equilibrium conditions.

In this connection, it is advantageous, when the components of therelative pressure sensor between the pressure chamber and the windingpath comprise a material of good thermal conductivity. Especially suitedhere are ceramics, especially aluminum oxide ceramics, as well ascertain metallic alloys. It is also advantageous, when the seams betweendifferent components of the relative pressure sensor exhibit a good heatconduction. The filter element is preferably likewise made of a metallicor a ceramic material with good heat conductivity, so that the filterelement assumes a sufficiently homogeneous and low temperature.Advantageously, the filter element is hydrophobic, or treated to behydrophobic.

For assuring a good heat conduction between the pressure chamber and thewinding path, there should be a sufficiently massive connection of heatconducting material between the pressure chamber and the path.Advantageously for this purpose, any cross section extending parallel tothe separating membrane between any point of the winding path and theplatform-side wall of the pressure chamber has a surface area fractionof heat conducting material amounting to at least 25%, preferably atleast 40% and especially preferably at least 50%, of the membranesurface area.

For assuring a good thermal contact between the atmosphere-side openingof the reference air path, respectively the winding path, on the onehand, and the pressure chamber, on the other hand, as small a separationas possible is provided between these elements. The separation of theplane of the winding path from the plane of the measuring membrane ispreferably smaller than the length of the winding path, especiallypreferably less than 75% of the length of the winding path and veryspecially preferably less than 50% of the length of the winding path.

For the relative pressure sensor of the invention, the followingvariants are among the possibilities for forming the winding path. Thewinding path can lie essentially in one plane, with a spiral shape beingpreferred. The plane of the winding path extends in such case preferablyparallel to the plane of the measuring membrane.

Winding paths with, for example, a helical shape are another option,with the length of the projection of the winding path onto the plane ofthe membrane amounting to at least 50%, preferably at least 65%, andespecially preferably at least 80% of the total length of the windingpath.

The winding path can include a line-shaped depression in a surface of acomponent of the relative pressure sensor, with the depression beingcovered with a suitable, additional component.

In another embodiment, the winding path can include a winding canal,which extends in at least one component of the relative pressure sensorbetween two openings in surface sections of the component. This can, forexample, be achieved by embedding a thread of an organic material duringthe forming of the green body, in order to predetermine the shape of thewinding path. This thread burns during the firing of the green body, sothat a capillary, winding path remains in place of the thread. Thewinding path can, for example, independently of the manner of itsmanufacture, have a cross sectional area of less than 2 mm², preferablyless than 1 mm² and especially preferably lie in the range of 0.7 to 0.4mm².

The winding path can, in principle, be arranged in the most varied ofcomponents of the relative pressure sensor. Possible components, amongothers, are the platform, or, for example, another component, which issecured to the platform. This other component can, for example, be alid, or a pot, which is placed on the platform and forms a preferablyhermetically-sealed chamber. Such a chamber can be suitable, especially,for accommodating the sensor electronics.

Preferably, the walls of the hermetically-sealed chamber in the pot arecoated with an electrically conductive material, so that the walls ofthe chamber are part of a Faraday cage, which surrounds the sensorelectronics and, if need be, other components of the relative pressuresensor.

For completing the Faraday cage, for instance, the lateral surfaces ofthe cylindrical platform of the relative pressure sensor, as well as therear side of the platform facing away from the measuring membrane,likewise are provided with a conductive layer. On the process side, theFaraday cage can be completed by an electrode applied to the internalside of the measuring membrane facing away from the process. Between theindividual components of the Faraday cage, of course, a sufficientlygood electrical connection is to be assured.

The conductive layers on the lateral surface of the platform, on itsrear side, on the walls of the hermetically-sealed chamber, as well ason the base surface of the pot, which is placed on the rear side of theplatform, can include a vapor-deposited or sputtered metal layer, aconductive foil or a conductive, sprayed lacquer.

Currently, sputtered metal layers are preferred, with Cu-containing,especially Cu—Ni-containing layers, being especially preferred.Exceptionally preferred are sputtered layers that were sputtered bymeans of a target exhibiting a Cu—Ni alloy. In the case of an alloyhaving more copper than nickel, the corrosion resistance rises withincreasing nickel. However, an increased nickel fraction can require anincreased soldering temperature. Additionally, layers having a nickelfraction that markedly exceeds the copper fraction prove to be unstable.Currently, Cu—Ni alloys are preferred to have a Ni-fraction of at least35%, preferably at least 40% and especially preferably between 42.5 and47.5%.

The layer thickness of the metal layer is not critical, with currently alayer thickness being 0.1 μm and 2 μm being preferred. Especiallypreferred is a layer thickness between about 0.5 μm and 1 μm, especiallyabout 0.7 μm. Optionally, Cr can be used as an adhesion promotor betweenthe ceramic and the conductive layer.

The conductive layer can either completely cover the rear side of theplatform facing away from the process, or it can be limited to an arealying outside of the base area of the hermetically-sealed chamber.

Similarly, the base area of the pot facing toward the platform canlikewise at least sectionally be coated with the conductive material, inorder to produce a conductive connection between the platform and thepot.

The mechanical connection between the pot and the platform of therelative pressure sensor can, for example, be created by a solder, aconductive adhesive, or the conductive coating itself. Especially suitedis a thixotropic epoxi-adhesive, for example that named HYSOL 9093,since, with this material, the roughness of the conductive layers on theceramic substrate is sufficient to assure an electrical connectionthrough the thickness of the adhesive coating.

As one skilled in the art will immediately realize, the just-describedaspect of the invention, namely the embodiment of thehermetically-sealed chamber being part of a Faraday cage, is not limitedto relative pressure sensors, since a climate protection andEMC-protection, such as is assured in such a chamber, is fundamentallyof interest for all types of pressure sensors.

As a result, the invention concerns also absolute pressure sensors andrelative pressure sensors, which are not used in a significantly humidsetting. Thus, the pot can also be designed without the above-describedreference air path and, in case a reference air conduit is required,this can, for less moisture-critical applications, occur, for example,by a capillary tube extending in the axial direction through the pot,with the tube not communicating with the volume of thehermetically-sealed chamber.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE is an exploded drawing of a relative pressure sensor ofthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The relative pressure sensor shown in the FIGURE includes a measuringcell 1, composed of a platform 10 and a measuring membrane 11 secured onthe platform and forming a pressure chamber therewith. The measuringmembrane 11 is loaded on its side facing away from the platform 10,during measurement operation, by the pressure being measured. Thepressure chamber communicates with atmospheric pressure through apressure chamber opening 12. The deformation of the measuring membraneresults from the difference between atmospheric pressure and thepressure being measured. The deformation can be registered by one of theestablished measurement principles, for example the capacitive,resistive or resonance methods. Corresponding electrical quantities areled through the platform passageways 13, 14, 15 out of the platform andprocessed by an electrical circuit (not shown). The circuit is arrangedon the platform 10 and covered with the pot 2, which is, for example,secured on the platform 10 with an adhesive having a good heatconductivity, so that the circuit is hermetically sealed and protectedfrom moisture. The electrical signals produced by the circuit are led tothe outside through passageways 23 opening on the end surface of the pot2 facing away from the platform. The end surface of the pot 2 exhibits aspirally-shaped depression 20, which forms a winding reference air path.The depression can, for example, have a v-shaped or semicircular crosssection. The cross section of the depression perpendicular to its lengthis about 0.5 mm². A first end of the depression 20 aligns in the axialdirection with the pressure chamber opening 12. A bore 22 through thepot 2 serves as a section of the reference air path between thedepression 20 and the pressure chamber. The pot 2 is, like the platform10 and the separating membrane 11, made of corundum ceramic. Thedepression 20 is produced by impressing a corresponding profile into thegreen body of the pot 2.

For assuring a good thermal contact between the pressure chamber and thereference air path, the pot 2 is made massive in that its material ismaximized, except for necessary cavities for accommodating theelectronic circuit and the passageways. In this massive design, thesurface area fraction occupied by material in any cross section throughthe pot parallel to the membrane amounts in this embodiment to at least50% of the area of the separating membrane.

To complete the reference air path, the depression is covered with acover 3, with such cover 3 being secured to the pot 2 with a connectionof good heat conductivity, for instance an adhesive. Aligning with thesecond end 21 of the depression 20 is an axially directed bore in thecover 3, this bore forming the atmospheric opening 31 of the referenceair path.

Arranged in the opening 31 is a filter element 4, which, in thecurrently preferred embodiment, is a hydrophobic, porous, ceramic filerelement. Similarly suitable are metallic filter elements, or organicfilter elements, for example those made of PTFE, with the filterelements being preferably hydrophobic or treated to be hydrophobic. Inthe spirit of good thermal contact, the filter element is secured in theinlet opening 31 with an adhesive of good heat conductivity.

Instead of the adhesive connections, fundamentally all other types ofconnection are suitable that enable a good conduction of heat.

Since the electronic circuit generates a small amount of waste heat, itsthermal contact to the reference air path, especially to the inletopening 31 of the reference air path, should be minimized. For thispurpose, the core region of the cover 3, which is axially aligned withthe circuit, is left vacant. Similarly, the central region of the endwall of the pot 2 is made thin, in order to minimize radial heatconduction.

1. A relative pressure sensor for measuring a pressure differencebetween a pressure being measured and the ambient atmospheric pressure,comprising: a platform; and a measuring membrane loadable with apressure being measured, wherein: said measuring membrane is secured atits edge to said platform; said pressure chamber is formed between saidplatform and said measuring membrane; and said pressure chambercommunicates over a reference air path with the atmosphere, saidreference air path includes a winding path.
 2. The relative pressuresensor as claimed in claim 1, wherein: said winding path liesessentially in a plane.
 3. The relative pressure sensor as claimed inclaim 2, wherein: said plane extends parallel to the plane of saidmeasuring membrane.
 4. The relative pressure sensor as claimed in claim3, wherein: the length of the projection of said winding path onto theplane of said membrane amounts to at least 50%, preferably at least 65%,and especially preferably at least 80% of the total length of saidwinding path.
 5. The relative pressure sensor as claimed in claim 1,wherein: the length of said winding path amounts preferably to at least75%, more preferably to at least 100% and especially preferably to 150%,of the length of the perimeter of said measuring membrane.
 6. Therelative pressure sensor as claimed in claim 1, wherein: the length ofsaid winding path is at least twice as long as the separation of anatmosphere-side opening of said winding path from the plane of saidmeasuring membrane.
 7. The relative pressure sensor as claimed in claim1, wherein: said winding path includes a line-shaped depression in asurface of a component of said relative pressure sensor.
 8. The relativepressure sensor as claimed in claim 1, wherein: said winding pathincludes a winding canal, which extends in at least one component ofsaid relative pressure sensor between two openings in surface sectionsof the component.
 9. The relative pressure sensor as claimed in claim 1,wherein: said winding path has a cross sectional area of less than 2mm², preferably less than 1 mm² and especially preferably n the range of0.7 to 0.4 mm².
 10. The relative pressure sensor as claimed in claim 1,wherein: the separation of the plane of said winding path from the planeof said measuring membrane is preferably smaller than the length of saidwinding path, especially preferably less than 75% of the length of saidwinding path and very specially preferably less than 50% of the lengthof said winding path.
 11. The relative pressure sensor as claimed inclaim 1, wherein: said winding path is in thermal contact with theplatform-side wall of said pressure chamber such that any cross sectionextending parallel to said separating membrane between any point of saidwinding path and the platform-side wall of said pressure chamber has asurface area fraction of heat conducting material amounting to at least10%, preferably at least 25% and especially preferably at least 50%, ofsaid membrane surface area.
 12. The relative pressure sensor as claim inclaim 1, wherein: said reference air path has a filter element at itsatmosphere-side inlet opening, for preventing the incursion ofcondensate into said reference air path.
 13. The relative pressuresensor as claimed in claim 12, wherein: said filter element is inthermal contact with said winding path.
 14. The relative pressure sensoras claimed in claim 12, wherein: said filter element is hydrophobic ortreated to be hydrophobic.
 15. The relative pressure sensor as claimedin claim 12, wherein: said filter element comprises one of: a ceramic,metallic and organic material.
 16. The relative pressure sensor asclaimed in claim 1, wherein: said chamber is hermetically sealed fromits environment; and additionally, at least one electronic component isarranged in said chamber.
 17. The relative pressure sensor as claimed inclaim 1, wherein: said winding path extends spirally or helically.